Vehicle navigation systems based on global positioning system (GPS) receivers have become pervasive in both business and recreational vehicles. Such vehicle navigation systems do not work well, however, when signals from GPS satellites are obscured or unavailable as may happen when driving in a parking garage or tunnel, in urban canyons, and/or near trees due to blocked lines of sight to the satellites or due to multipath propagation.
Accordingly, dead-reckoning is increasingly being implemented in vehicle navigation systems in order to overcome the problems associated with GPS technology alone. In navigation, dead-reckoning is the process of calculating a current position by using a previously determined position, or fix, and advancing that positioning based upon known or estimated speeds over elapsed time and course.
In a vehicle navigation system that uses dead-reckoning, the vehicle is equipped with inertial sensors (e.g., accelerometers and gyroscopes) that know the velocity and direction of travel of the vehicle. Accordingly, such a vehicle navigation system that uses dead-reckoning can be implemented as a supplement to or in lieu of a GPS-based vehicle navigation system. For example, a vehicle navigation system may use a Kalman filter to integrate the always-available sensor data with the accurate but occasionally unavailable GPS position information into a combined fixed position. Alternatively, a vehicle navigation system that uses dead-reckoning can provide navigation data in the event of GPS signal loss.
The sensors used for dead-reckoning typically require recalibration based on GPS velocity and position information. Indeed, in the absence of recalibration signals, a vehicle navigation system that uses dead-reckoning quickly accumulates velocity and position errors due to drift, noise, and a random-walk drift tendency of the accelerometer data that are at levels unacceptable for the purpose of vehicle navigation. Accordingly, periodic recalibration based on GPS data can reduce accumulated position and velocity errors when operating in a dead-reckoning mode utilizing the supplemental inertial sensor measurement system. However, continuous recalibration of the vehicle inertial navigation data based on the GPS data requires sufficient frequency of such recalibration events and, as such, requires high availability of the GPS signal.